Wide frequency band amplifier



Oct. 10, 1950 w. A. MoNTGoMERY wrm: FREQUENCY BAND AuPLI'x-IER 3 SheetsSheet 1 Filed March 5, 1945 F/GS.

Oct. 10, 1950 w. A. MONTGOMERY WIDE FREQUENCY BAND AMPLIFIER 3 Sheets-Sheet 2 Filed llarcb 5, 1945 Inventor 0d- 10, 1950 w. A. MoNTGoMERY 2,524,821

wInE FREQUENCY BAND AMPLIFIER Filed llaroh 5, 1945 3 Sheets-Sheet 3 Inventor Patented Oct. 10, 1950 2,524,821 WIDE FREQUENCY BAND AMPLIFIER William Alan Montgomery, London, England, as-

sgnor, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 5, 1945, Serial No. 581,098 In Great Britain December 28, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires December 28, 1963 6 Claims. l

The present invention relates to multi-stage amplifiers employing thermionic valves of the grounded-grid type and is concerned with the interstage coupling arrangements for such amplifiers, particularly those designed for use at ultra-high frequencies.

The valves are of the kind intended to be operated with the control grid connected to earth, or at least maintained at earth potential for the signal waves. The input signal voltages are therefore applied to the cathode, and the-output voltages are taken between the anode and control grid. The valve operated iri this way has a very low input impedance, and the reactances associated with the output circuits of the valves may have a controlling inuence on the coupling arrangements of two successive stages. Furthermore, since the input impedance of the valve is low, the coupling arrangements must full certain definite conditions if maximum gain over the transmitted band is to be obtained, which conditions are quite different from those which apply when the valves are operated in the usual way.

The principal object of the present invention is to provide an eicient coupling network for two stages of a grounded-grid valve amplifier intended to pass a specified band of frequencies. While the frequencies of special interest to the invention are in the ultra-high-frequency range, the same principles are applicable in other ranges. y

According to the invention there is provided an electric wave amplifier comprising two thermionic valves having their control grids connected to ground. and a network connecting the anode of the first of the said valves to the cathode of the second valve, the said network being so designed that when combined with the inter-electrode capacities of the valves it constitutes a transforming filter adapted to pass a frequency band of specified width, the output image admittance of the said filter at the mid-band frequency being equal to the conductance component of the cathode-control grid admittance of the'said second valve at that frequency, when the anode of that valve is connected to a network similar to the said network or to an equivalent load.

In a rather more specific form, the invention also provides an electric wave amplifier comprising two thermionic valves having their control grids connected to earth, a system of hollow metal tubes enclosing the said valves and having central conductors connected in such a manner as to form with the inter-electrode capacities of the said valves a transforming iilter for coupling the anode of one valve to the cathode of the other valve, the said filter being adapted to pass a band of frequencies of specified width, and having an outputiimage admittance at the mid-band frequency equal to the conductance component of the cathode-control grid admittance at that frequency of the said other valve when the anode of that valve is loaded by a similar transforming filter, or by an equivalent load.

The invention will be described with reference to the accompanying drawings in which:

Fig. l shows a schematic circuit diagram showing two grounded grid valves coupled by an arrangement according to the invention;

Figs. 2 to 6 show circuit diagrams of lters to show how a transforming filter according to the invention may beevolved Fig. 7 shows a schematic circuit diagram of two stages of an amplier employing a lter according to Fig. 6;

Fig. 8 shows an alternative configuration for the transforming filter;

Fig. 9 shows a schematic circuit diagram of two stages of an amplifier employing a filter according to Fig. 8; and

Fig. l() shows an embodiment of an amplifier employing a filter having the configuration of Fig. 5.

Figure l1 shows another embodiment employing a filter having the configuration of Figure 6.

The invention is not restricted to amplifiers employing simple grounded-grid triodes only. There indy be any number of electrodes, and the valves may be of special types, such as beam tetrodes or aligned-grid tetrodes, provided always that the control grid is earthed. However, triode valves having three parallel plane electrodes specially designed for ultra-high frequencies are of particular interest in connection with the present invention. Such valves are described, for example, in the specification of U. S. Patent No. 2,419,544.

In coupling valves together, the reactances associated with the input and output circuits of the valve, and with the connection leads, must be taken into account and may be controlling factors, and `it has been found that these reactances ,can be utilised to form the elements or parts of the elements of a band-pass coupling filter designed to pass with a minimum of distortion a certain band of frequencies. As has already been mentioned, the input impedance of a grounded grid valve is rather low, and therefore in order to obtain a coupling which operates with maximum eiiiciency, the coupling filter should include the equivalent of a step-down transformer. In other words, an impedance transforming filter iS required.

The reasons for the particular choice of the characteristics of the coupling filter according to the invention will now be explained.

Let it be the amplification factor of the type of grounded-grid valve chosen for use. Let R be its internal impedance, that is, R=(E+;1e)/i, where E and e are the instantaneous values of the A. C. components of the anode-cathode and control grid-cathode votages, and z' is the corresponding instantaneous value of the A. C. cornponent of the anode current. It can be shown by simple circuit theory that the input impedance Z1 measured between the cathode and control grid is given by:

where Zt is the external load impedance connected between the anode and control grid of the valve. This is on the assumption that there is no grid current, that the anode-cathode capacity of the valve is negligible, and that the cathodecontrol grid and anode-control grid capacities C1 and Ci are considered as'parts of the external circuitsconnected to the valve.

Fig. 1 shows in block schematic form a circuit according to the invention. There are shown two simiar grounded grid valves V1 and V2 coupled by a network N of reactive elements which together with the Valve capacites Ct and C1 forms a transforming filter TF. This filter is terminated on its output side by the impedance Z1, and Zt is the impedance measured at its input terminals when so terminated.

Assuming that the anode-cathode capacity of the valve is negligible, so that no direct feedback occurs between the anode and cathode circuits, it is easy to show that the power gain introduced by the valve at any frequency is determined by the radio G of the real part of Zt to the real part of Z1. This can be seen because the current flowing in at the cathode (into Z1) must be the same as the current flowing out at the anode (into Zt) it being assumed that there is no grid current.

It will be understood from what has already been explained, that according to the invention, the capacities Ct and C1 are treated as though they formed part of they coupling filter TF. These capacities will be taken to include any additional stray capacities introduced by leads, and the like, which will, of course, be reduced to a rilinimum. They will therefore be neglected for the present when considering the action of the valves, which will thus be assumed not to have any capacities associated with them.

The design of the filter TF' will be treated rst of all in terms of the mid-band angular frequency wo which is equal to Van-wz, where w1 and wz are the two cut-ofi angular frequencies.

From Equation 1 it can be seen that Z1 is real if Zt is real, and vice-versa. Let it be assumed that Z1 is real vand equal to R1. The coupling filter TF will be designed according to the invention so that its output image impedance at wo is R1. Then its inout image impedance will be R1/n2 where n is 1ess than 1 and is the transformation ratio of the equivalent'steri-down transformer effectively contained in the filter. Thus and is real, so that the condition of Equation 1 is satisfied. Thus substituting in Equation 1 we have 'I'hus G increases as Re increases, and the maximum theoretical value of G is therefore ,1+1, but only much smaller values are usually possible for reasons which will a-ppear later.

In order to fulfill these requirements it will be necessary to provide an anode load equal to Rt at the mid-band angular frequency we for the last amplifying valve of the series, then the anode load of all the other valves will be Re as desired.

The properties of .various types of band-pass filters are summarised in Fig. 168A, B and C of the book Transmission Networks and Wave Filters by T. E. Shea. In view of the desirability to include the capacities Ct and C1 as parts of the fiter, it is necessary to choose a configuration which can have shunt capacities at the ends. Also, in order to obtain the maximum gain, Rt should be as large as possible, and this requires that the input shunt capacity of the lter should be as small as possible. The smallest possible value of this capacity is Ct, .the unavoidable anode-control grid capacity of the valve. By reference to Fig. I168A, B and C of the work mentioned above, several of the simpler configurations are suitable, and of these Nos. IVk and III: give the largest band-width wis-wi for given values of Ct and Rt. Practically all the others give a smaller bandwidth and are therefore less suitable. Of the two selected, configuration Ill-1. is the simpler and is the preferred configuration according to this invention. However, other forms could be used, if desired.

The configuration IIL is shown in Fig. 2, to which has been added an ideal transformer T to introduce the ratio n, in which the components indicated have theV following values:

Since the transformer has been introduced on the output side of the lter, the input characteristic impedance of the filter is Rt, and it will be properly terminated by the impedance R1 connected on the other side of the transformer. It will be understood from what has already been explained, that R1 is provided by the input circuit of the valve V2, from which circuit the capacity C1 has been removed and considered as part of the filter. Thus the admittance l/R1 is the same as the conductance component of the admittance of the input circuit of the valve, which admittance could, of course, only be measured with the capacity C1 present. Likewise, although l/Rt is the input image admittance of the filter, it would most probably be measured as the conductance component of the admittance looking into the input side of the network N (Fig. 1) but the capacity Ct could be included by making the measurement with the valve V1 in position, but with the heater switched off.

The transformer T may be incorporated into the network by the series of well known transformations shown in Figs. 3, 4 and 5. In Fig. 3 it has been moved to the left-hand side of La and C2, so that these elements are changed respectively to L3=1L2L2, and C3=C2/n2. The transformer combined with L1 is equivalent to the three inductances L4, L5 and Le shown in Fig. 4, the values of which are L4=n2L1/(1n), L5=nL1, and Ls=nL1/(n-1).

that Le is negative, since n is less than 1.

It will be noted for Fig. 6:

By combining the inductances Le, L4 and La, Le, the iilter of Fig. is obtained, in which:

If Ls is to be physically realisable it must be positive, and therefore L2 must not exceed nLi/ (l-n) The prefered design procedure for the filter is as follows:

C2 is first taken equal to Ct, the anode grid capacity of the valve together with the capacity of any Aleads attached thereto. The speciiied band Width wz-wl then determines Rt from Equation 6. The inductances L1 and Le are then determined from Equations 4 and 5 by inserting the specified values of w1 and wz. Having determined the -value of n then L5, Lv and La are found from the Equations '7.

It will be seen from Equation 2 that n is fixed by the choice of the valve, when Rt has been determined; thus it may sometimes be found that La is negative and therefore unrealisable. This means that with this particular type of filter designed in the manner explained, when the band Awid-th is specified, the mid-band angular frequency wo cannot be below a certain minimum value, though it can be above. This is not a serious disadvantage in some amplifiers, because that La is infinite, and is'therefore omitted altogether. The filter then reduces to the form shown in Fig. v6. This requires that:

L2=nLi/(1n) (8i From Equations 4, 5 and 8, it then follows that I'his determines w1 and wz separately since the bandwidth u2-w1 is given, and the values of L1 and Lz are then found from Equations 4 and 5, and thence L5 and L7 from Equations '7. The filter then reduces to the simplest form shown in Fig. 6. Since C3=C2/n2, it will usually be much larger than C2, and will also generally be several times larger than C1, so that C1 does not limit the design, and some extra capacity must be' added to make up C3.

The method of designing a network according to Fig. 6 will be better understood from a numerical example. The grounded grid valve chosen for use had the following constants:

From Equation 6, the value of Rt is found to be about 2340 ohms, by putting C9=Cn from Equation 2 n2=0.0945, and thus Ri=221.2. From Equation 9, since (wa-w1)/21r=40 megacycles, it follows that col/2W 107 i2/2r 147 megacycles per second io/2r 125.4

From Equation 4 L1=2.933 microhenries, and thus L5=0.902 microhenry and L1=0.094 microhenry. Also Ca=C2/n2=18.0 micro-microfarads. Also G=1/n2=10.58, corresponding to a gain of about 10.2 decibels.

It will thus be seen that C3 is about ve times C1, so that an additional capacity of about 14.6 micro-microfarads must be connected across the output of the filter TF.

It will of course be found that the values of C: and Ci for a number of different valves of the same type vary between certain limits. In order that the transforming lter shall give the desired lperformance with any valve, a small variable trimming condenser may be connected in parallel with C2. The filter should be designed for a value of Ct equal to the maximum value for any valve, and this variable condenser is adjusted so as to bring Ct up tothe maximum value for the particular valve employed. Similarly, vari-ations in Ci will be covered by using a variable condenser to supply partly or wholly the additional capacity (about 14.6 micro-micro farads in the example given above) required to make urp Ca.

Referring again to Fig. 5, the star formation equivalent to the delta form of the three inductances L5, In and Ls may of course be used if preferred, as it may be more convenient in some circumstances. It will, however, suffer from the same limitation as regards the minimum value of wo, and in the special case when Equation 8 holds both forms are identical.

The design of th'e filter has so far been based upon consideration of the mid\-band angular frequency wo at which the filter has an image impedance which is a pure resistance, and the filter can be and is terminated correctly by a pure resistance at this frequency. These conditions do not generally hold at other frequencies in the pass band when the filterv is so terminated, and the gain calculated from the simple formula G--1/n2 does not generally hold at other frequencies. It can, however, be shown that for the type of filter shown in Figs. 2 to 6, the gain at w1 and wz is the same as that at wu', but between wi and wo and between wo and wz the gain is generally a little higher, there being two unequal maxima, one on either side of wo. It is however, unlikely that the maximum gain in any Apart of the band will be more than about 1.6 db. above the gain at wo.

Although the maximum gain per stage will be obtained by making C2=Cc, a larger value of C2 could be chosen if desired, whereby some freedom of design could be obtained at the expense of some reduction in the gain. Ihis would give a lower input impedance for the filter and a larger value of n.

The gain per stage which it is possible to obtain is in effect limited by the characteristics of the valve chosen, and varies opfpositely with the band width; thus wider bands imply lower gain. For valves having the same value of p. the available gain per stage is greater when R and Ct are smaller. Thus the valve should have high values of ,L and low values of Ct and R.

Fig. 7 is a schematic circuit diagram to show how the coupling filter of Fig. 6 may be applied in practice. The capacities C1 and Ct associated with the valves are shown dotted in order to indicate that they do not represent any actual circuit elements. The condensers designated K are bypass condensers of relatively large capacity so that their reactance at the operating frequency is negligible. The anode of V1 is connected to the cathode of V2 through the coil L5 and a blocking condenser H. Anode potential is supplied to the anode of V1 through the shunt coil I nand through L5, the coil Inbeing effectively connected to earth through the bypass condenser K. The variable condensers Qt and Q1 are connected in parallel with C1. and C1 and enable the desired values of the filter capacities Cz and Ca to be obtained in the manner described. Q1 is preferably a very small condenser having a range just suicient to cover the maximum variation of C1.

The cathode heaters are supplied from separate heating sources HSI and HS2 through choke coils Lh'having a very high impedance at the operating frequency in order to prevent the heating source from short circuiting the filter. These coils together with the corresponding bypass condensers K also prevent coupling between the stages through the heating sources. Cathode bias is provided by the resistances Rb. The inductances L5 and L1 may consist of simple solenoids of a few turns and should be placed with their axes at right angles and not too near together in order to avoid any appreciable mutual inductance which would modify the action of the lter. The coils L11 may also be solenoids not too closely wound in order to reduce the self capacity. Each of the condensers K can usually be provided by means of a small plate fixed to the screen of the amplifier and insulated therefrom by a sheet of mica or the like. Qt and Q1 can be small rotary air condensers of conventional type. y

The filter according to Fig. 5 may be constru-cted in another way. It is well known that the network of the three inductances L5, In and La is equivalent to a two-winding transformer. This figure can therefore be redrawn as Fig. 8, in which In and L are the inductances of the two transformer windings, and M is the mutual inductance between them.

The equivalence of these two networks is discussed in the article by E. K. Sandeman entitled Coupling Circuits as Band Pass Filters in the Wireless Engineer, vol. XVIII, No. 216, September, 1941, pages 363 to 365. From the formulae given'in that article, it follows that for the vcon. ditions of the present case:

C2 and Ca having the same values as before. It is to be noted that realisable values of La, L10 and M can be chosen for any desired values of w1 and m so that the restrictions inherent in the network of Fig. 5 do not apply to Fig. 8. The values of n and Rt are of course determined when C1 and ua-wa are specified, just as in the case of Fig. 5, and the maximum gain obtainable is the same.

If the same numerical case as before be taken, using the same type of valve, then 11.2=0.0945

and Rt=2340 ohms, w2/21r=147 and w1/21r=107; then substituting in Equation 10 itis found that:

L9=0.996 microhenry Lro=0.0941 microhenry M=0.0940 microhenry the values of C; and Ca being the same as before.

In order to illustrate a case in which the filter of Fig. 5 would be unrealisable, while Fig. 8 would be possible, suppose that a wider band be taken, and let 11/21r=100 megacycles and wz/21r=150 megacycles the same valve being used. From Equation 6 Rt=l872 ohms, and from Equation 2 n2=0.1156

giving a gain of about 9.4 db. instead of about.

10.2 db: for the narrower band. Thence from Equations 10 Ln=1.076 microhenry VLw=0.1245 microhenry M=0.1408 microhenry ments which are the same in both figures are similarly designated and will not be again described. In Fig. 9, the anode voltage for the valve V1 is supplied through the winding L9 of the transformer, and current for the cathode heater is fed through L10 so that only one choke coil L11 is needed for the other heater lead. Also no blocking condenser corresponding to H in Fig. 7 is required. r

The network of Fig. 5 may also be realised in a different way by supplyingv the inductance eiements in the form of short sections of co-axial transmission lines. The manner in which this may be done is fully explained in the specification of U. S. Patent 2,284,529. The configuration described withreference to Fig. 2 of that specification would be suitable in the case of the present invention. Fig. 6 might also be suitable, since it produces the star arrangement of the three inductances which is equivalent to the delta arrangement of the accompanying. Fig. 45. It is pointed out in the specification referred to that the lengths of the line sections used should be less than about one eighth of the shortest wavelength evolved so that the line sections operate substantially as lumped inductive elements. The necessary formulae for dimensioning the line sections are given in U. S. Patent 2,284,529 so they will not be quoted again here.

The accompanying Fig. 10 shows the manner in which the transmission lines arranged as in Fig. 2 of Patent 2,284,529 may be applied to thepresent invention. The two valves V1 and V2 are arranged inside a tube I, only part of which is shown. 'I'he valves are preferably of the type having a disc terminal for the grid, and this is arranged so that the grid with its disc forms substantially a perforated partition across the tube. The anode of V1 is connected to the cathode of V2 by a conductor 2 coaxial with the tube i and divided by a blocking condenser H. This constitutes the inductance L5 of Fig. 5. Two side tubes 3 and 4 are provided, the central conductor of tube 3 being connected to the anode end of conductor 2 and passing out of the tube 3 at the closed end. A plate 6 insulated from the end of the tube forms a bypass condenser. The conductor 5 is connected to the anode supply source. The central conductor of the tube 4 is a tube 1 connected to the cathode end of the conductor 2, and the second heater conductor 'B passes through this tube and out through theclosed end of the tube 4. A flange 9 attached to the tube 'I forms the necessary bypass condenser, and a lead I0 connected to the flange 9 also passes outside the tube 4. Leads 8 and I8 are connected to the cathode heatingr source HS. The condensers Qt and Qi are supplied as before, and are connected between the wall of the tube I and the anode and cathode, respectively. A resistance II is provided for biassing the cathode of V2.

The tubes 3 and 4 with their central conductors constitute the inductances Ls and L7 of Fig. 5.

In the case where there are several more similar amplifying stages, all the valves may be assembled inside the tube I, each pair of successive valves being connected by an. exactly similar arrangement of two side tubes like 3 and 4, and a central conductor similar to 2 containing a blocking condenser H.

If it is desired to reproduce in this form the filter of Fig. 6, the side tube 3 is omitted, as shown in Fig. 11. The central conductor I of the side tube 4 is connected to the central conductor 2 which in this case has no Ablocking condenser dividing it into two parts. The high tension for the anode of V1 is supplied through the conductors 'I and 2. Both the cathode heater leads I0 and I2 for the valve V2 are passed through the conductor 1, but both ends of the heater are coupled thereto by bypass condensers K. The remaining elements of Fig. 11 which have not been mentioned are the same as in Fig. 10.

What is claimed is:

1. An electric wave amplifier comprising two thermionic tubes each having a cathode and an anode, a hollow conductor having said tubes mounted therein, a grounded grid in each of said tubes directly connected to said hollow conductor and forming a substantially perforated partition thereacross, two conductors mounted insde said hollow conductor between said tubes and forming a coaxial conductor line with said hollow conductor, a direct current blocking xcondenser connecting said two conductors together, said two conductors being directly connected one to the anode of one of said tubes and the other to the cathode of the other of said tubes, and two f quarter wave coaxial line type stubs connected across said coaxial conductor, one adjacent said one tube and the other adjacent said other tube, said stubs being dimensioned to inductively load said coaxial conductor, said coaxial conductor being dimensioned to couple inductively said tubes together, the input and output inter-electrode capacities of said tubes forming with the coaxial elements a transforming filter adapted to pass a frequency band of given band width.

2. An amplier according to claim 1 in which 10 the inner conductor of each of said coaxial line type stubs is insulated from the outer and directly connected one to the anode of said one tube and the other to the cathode of said other tube for applying operating potentials thereto.

3. An amplifier according to claim 1 in which additional capacities are respectively connected in shunt between the anode and grid of said one tube and between the cathode and grid of each other tube.

4. An electric Wave amplier comprising two thermionic tubes each having a cathode and an anode, a hollow conductor having saidy tubes mounted therein, a grounded grid in each of said tubes directly connected to said hollow conductor and forming a substantially perforated partition thereacross, two conductors mounted inside said hollow conductor between said tubes and forming a coaxial conductor line with said hollow conductor, said two conductors being directly connected onev to the anode of one of said tubes and the other to the cathode of the other of said tubes and a blocking condenser coupling said two conductors together, a coaxial line type stub connected across said coaxial conductor line, said stub being dimensioned to inductively load said coaxial line, said coaxial line being dimensioned to inductively couple said tubes, the input and output inter-electrode capacities of said tubes forming with the coaxial elements a transforming filter adapted to pass a frequency band of given band width.

5. An amplifier according to claim 4 in which the inner conductor of said coaxial line type stub is hollow and insulated from the outer conductor thereof and directly connected to the anode of said one tube for applying an operating potential thereto, and an additional conductor is arranged inside said hollow conductor and directly connected to the anode of said other tube for applying an operating potential thereto.

6. An amplier according to claim 4 further comprising additional capacities connected in shunt between the anode and grid of said one tube and between the cathode and grid of said other tube.

WILLIAM ALAN MONTGOMERY.

REFERENCES CITED The following vreferences are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,896,534 Alexanderson Feb. 7, 1933 2,107,387 Potter Feb. 8, 1938 2,143,671 Zottu Jan. 10, 1939 2,149,356 Mason Mar. 7, 1939 2,284,529 Mason May 26, 1942 2,321,521 Salinger June 8, 1943 2,419,800 Tomlin Apr. 29, 1947 2,426,185 Doherty Aug. 26,1947

OTHER REFERENCES Article by Jones, Proceedings of the I. R. E., July 1944, vol. 32, No. 7, pp. 423-429, reprint in 179--171-1. 

